He I absorption lines in high-redshift Lyman limit systems of the QSO HS 1700+6416.

Abstract

We report the detection of He I resonance lines in four high- redshift Lyman limit systems of the QSO HS 1700+6416 (z = 2.72) observed with the Faint Object Spectrograph on board the Hubble Space Telescope. We find neutral hydrogen to neutral helium column density ratios N_HI_/N_HeI_ ~ 30 (possible range 20 to 70). Ionization calculations using CLOUDY (Ferland 1991) and UV background radiation fields, empirically gauged with heavy element ionization ratios in the same redshift systems, yield He I column densities lower by a factor of 5 than observed column densities. We briefly discuss reasons for this discrepancy. HeI as minor ionization species (HeI/He ~ 10^-4^) is apparently not useful for determining the He abundance in high redshift clouds. We do not see HeI lines in Lyα forest clouds at the FOS resolution, as expected according to ionization calculations. However, in case of cool, dense, mainly neutral clouds, as proposed by Pettini et al. (1990) the HeI lines could be detectable by future HST observations of this QSO, and HeI upper limits can be used to infer lower limits on the Lyα cloud temperatures and sizes. We briefly discuss the possible range of strengths of the He II 304A forest.